The procedures are
continually being updated to standardize inspection procedures
across all models and to provide more explicit inspection content.
Currently, quantitative vertical acceleration thresholds are
being added to all Boeing-designed airplane conditional inspections
to assist operators in deciding whether to initiate hard landing
inspections. Similar information is being added to the AMMs
for Douglas-designed airplanes. Boeing also is adding qualitative
data to the 737 AMM conditional inspection maintenance procedures
based on operators’ requests and service experience. Although
specific to 737-100/-200/-300/-400/-500, the rationale and some
of the data may be applied to other Boeing airplane models.

Boeing is updating its
conditional maintenance procedures for 737 non-normal landings,
such as a hard landing, a high-drag-load or side-load landing, an
off-runway excursion, or a tail strike. The changes are based largely
on airline service experience (see Non-Normal
Landing Service Experience) and are part of an
ongoing effort by Boeing to update recommended unscheduled inspections
after non-normal events for all its models. The changes involve

Flight
crew judgment. Boeing commercial airplanes have been
designed for a 10-ft/s sink rate at or below the maximum design
landing weight and a 6-ft/s sink rate at more than the maximum design
landing weight. A hard landing inspection is recommended if these
values are approached or are exceeded. Because the sink rate is
not directly measured, however, the flight crew must rely on its
own judgment or peak vertical center-of-gravity (CG) acceleration
from the flight data recorder (FDR) after the flight to determine
whether an inspection is warranted.

Service experience indicates
that most flight crews report a hard landing when the sink rate
exceeds approximately 4 ft/s. Past experience also indicates that
the flight crew’s determination of a hard landing is the most reliable
criterion because of the difficulty in interpreting recorded acceleration
values at the CG of the airplane.

Vertical
acceleration values. Using vertical acceleration values
as the sole criterion for initiating unscheduled inspections is
generally not advisable because of the location and design considerations
of the FDRs and accelerometers. In most instances, there is no absolute
way of knowing whether the recorded accelerations are a minimum,
maximum, or some intermediate value relative to the entire airframe
structure. This is because the onboard accelerometer located near
the airplane CG is limited in its capability to capture actual loads
that may be occurring in the entire airplane structure during the
landing impact.

Several accelerometers
placed throughout the airplane have shown significant variations
in both time and magnitude of vertical acceleration values, or structural
loads. These variations are the result of airplane weight, CG, motion
(e.g., sink rate; forward and side velocity; roll, pitch, and yaw
angles; and corresponding rates), external forces (e.g., gust loads,
ground effect, and runway contact loads), and structural dynamics
(e.g., vibrations and harmonics). Also the sampling frequency of
the recorded vertical acceleration data  which is subject
to the specific flight recorder installation and varies from 4,
8, or 16 samples per second  can cause wide variation in recorded
peak vertical acceleration values.

After reviewing some
limited data generated by operators, Boeing decided to perform more
extensive analysis to correlate vertical acceleration with the design
sink rate. Figure
1 shows an example comparing the recommended inspection
threshold with analytical digital FDR accelerations (based on an
eight-samples-per-second digital FDR) for the 737 family at the
design sink rate of 10 ft/s. The figure compares the peak CG acceleration
with the filtered digital FDR sensor acceleration (both the calculated
peak and the lowest possible recorded peak value at eight samples
per second). The inspection threshold is slightly less than the
lowest data point to minimize the number of nuisance inspections
without exceeding the design sink rate.

With this information,
Boeing developed vertical acceleration thresholds to trigger operator
review of flight data when these acceleration values are exceeded
in service. The thresholds, in addition to flight crew judgment,
give operators the option of initiating AMM conditional inspections
after review of FDR data.

AMM
changes. Boeing is modifying the 737 AMM, section 05-51,
and will revise the AMMs for other Boeing-designed airplane models,
to include these vertical acceleration values (table
1). For Douglas-designed airplanes, similar values are
being generated and will be available in the AMMs in early 2001.
The values are intended as thresholds that can be used to help determine
whether a hard landing inspection is necessary. If the flight crew
concludes that it has experienced a hard landing, the AMM conditional
inspections should be performed even if the acceleration readings
do not exceed the values added to the AMM. The vertical acceleration
values are to be used by operators, in addition to or in lieu of
flight crew judgment, to initiate conditional maintenance inspections.

The values shown in the
AMM have a relatively low threshold to ensure that the airplane
has not exceeded the design sink rate and that the structural integrity
of the airplane has been protected. This may lead to some unnecessary
inspections; however, it should reduce the number of inspections
performed overall because many inspection thresholds established
by airlines at this time are for very moderate sink rate landings.
Upon retrieval of flight data, when certain CG acceleration thresholds
are exceeded, operators can decide when their maintenance crews
should perform a hard landing inspection.

Note that if the acceleration
values are recorded during a hard nose landing or accompanied by
more than 2 deg of roll at the time of main landing gear impact,
a hard landing may be experienced at significantly smaller vertical
acceleration values. This information is included in the AMM revisions.

Data
retrieval. In addition to the FDR, many airplanes have
an optional onboard maintenance data system. Maintenance data can
be more easily retrieved from such a system than from the FDR.

One type of onboard maintenance
data system is the airplane condition monitoring system (ACMS) installed
on many 737, 747, 757, 767, and 777 airplanes. ACMS is hosted in
either the data management unit (DMU) or the digital flight data
acquisition unit (DFDAU) on 737, 757, and 767 airplanes and in the
DMU on 747s. It is provided by the airplane information management
system (AIMS) on the 777. Three companies build DFDAUs with ACMS
capability that can be tailored to meet each operator’s unique needs:
Honeywell Aerospace, Redmond, Wash., USA; SFIM, Paris, France; and
Teledyne Technologies, Inc., Los Angeles, Calif., USA.

Bulk flight data from
the ACMS are stored in the quick-access recorder (QAR), located
in the electrical and electronic equipment bay on the airplane.
The QAR operates much like the floppy disk drive on a desktop computer.
It uses a standard optical disk or solid-state memory card as the
storage media. In service, the operator takes the QAR disk from
the airplane and loads the data into a maintenance ground station
computer for further processing.

Because the ACMS is not
required for airplane certification, the operator determines which
information to record. Some operators find the FDR data sufficient
for their maintenance program and put the same data on both the
QAR and the FDR. Other operators specify a different set of parameters
for the QAR for more detailed performance data on systems that tend
to drive maintenance costs such as engines and on operational practices
during landing, takeoff, and taxi.

In addition to using
flight data for unusual flight and landing conditions, operators
may use the data to evaluate maintenance requirements by collecting
QAR data weekly and keeping permanent records. This can be done
for an operator’s entire fleet or a sampling of airplanes.

Operators review these
data to see whether changes in operation coincide with changes in
frequency of conditional maintenance. For instance, in the absence
of flare during landing, there is a tendency for three-point landings
and generally higher sink rates at touchdown. If severe enough or
done often, this can damage or wear nose landing gear components.
This damage or wear usually is found at scheduled gear overhaul.
Initial touchdowns on the main gear during flared landing allow
the airplane to straighten out before nose gear contact, such as
in crosswind conditions. This will avoid putting side loads on the
nose gear at high speeds and create less wear on some nose gear
components, such as tires, steering components, and internal shock
strut components, including upper and lower bearings, centering
cam components, and their anti-rotation devices.

2.
MORE EXPLICIT INSPECTION
PROCEDURES

In addition to vertical
acceleration thresholds being added to the AMMs, the conditional
maintenance inspections are being updated in the 737 AMM to provide
more explicit inspection procedures. The conditional inspections
are constantly being updated and geared toward ensuring continued
serviceability and structural integrity of the airplane.

Changes to the 737 AMM,
section 05-51, involve the conditional maintenance inspections for
hard, high-drag-load, and side-load landings, as well as off-runway
excursions. These changes will permit operators to make the most
effective use of the two-phased conditional maintenance inspection
process. The changes also will keep phase I inspections as simple
as possible with minimal access and disassembly requirements.

The two-phased inspection
starts with a close visual inspection of various structural components,
especially those most vulnerable to damage, to determine whether
further inspections are warranted. A second phase of inspections
is conducted if any damage is found during phase I.

In some instances, operators
may request that Boeing review their findings if structural damage
or fuel or hydraulic leakage is detected during phase I or phase
II inspections. Boeing also may be asked to review flight recorder
data. (See Analyzing
QAR Data.) The reviews may indicate that further
inspections are warranted.

AMM
changes for non-normal landings. Changes to the unscheduled
maintenance procedures in the 737-100/-200/-300/-400/-500 AMM will
involve both phase I and II structural inspections. The most significant
changes will involve inspection of the main and nose landing gear
and supporting structure.

These changes are as
follows:

During the phase I inspections
of the main landing gear, operators should check for shock strut
leakage and examine the inside diameter of the fuse pins of the
drag strut and the outboard end of the main landing gear beam for
distortion. This involves checking for visible damage to the specific
component without removing it.

Operators also should
examine the main landing gear beam to the inboard rear spar stabilizing
link for damage to the link or the crank shafting of the forward
and aft attach bolts (figs.
2 and 3).
This is accomplished by loosening the nut on the stabilizing link
bolt and turning the bolt to determine whether it is deformed or
crank-shafted.

Damage at this location
will warrant further action during phase II inspections; specifically,
the trunnion link should be removed in accordance with the AMM and
the forward trunnion fuse bolt inspected (fig.
4). On the outboard attach fuse pin for the main landing
gear beam, the retention bolt should be removed and the pin rotated
to check for crank shafting.

During phase II inspections
of the main and nose landing gear, operators should ensure proper
hydraulic fluid levels are in the shock struts by performing a two-point
service check, or by completely servicing the shock struts in accordance
with the AMM. Operators also should remove the landing gear inner
cylinders if shock strut servicing was found to be incorrect or
if both a hard and a high-drag-load or side-load landing occurred
at the same time. The barrel of the inner cylinders and axles also
should be dimensionally checked for distortion or bending and examined
for cracking.

Airline technical and
operational staff may be consulted following phase I and II inspections,
depending on inspection findings. Boeing is often requested to provide
technical assistance during such reviews.

AMM
changes for off-runway excursions. Off-runway excursions
occur either on hard, even surfaces that do not create higher-than-normal
loads or on uneven surfaces with depressions and obstructions that
also may include soft and muddy conditions. The latter situation
can create high vertical, high drag, and side loads when the gear
goes over rough terrain or when the airplane stops suddenly in soft
terrain.

Maintenance procedures
for off-runway excursions are being added to the AMM for the 737-300/-400/-500
and revised for the 737-100/-200 AMM as follows.

Travel onto surfaces
with depressions or obstructions will generally require close inspection
of all fuse pins during the two-phased inspection process outlined
in the AMM. The gear then may be removed for closer inspection depending
on flight crew judgment, FDR/QAR data review, consultation between
the operator and technical experts, or the discovery of any structural
anomalies. In addition to fuse pin deformations, axle and truck
deformations may be discovered during close inspection of the gear.

When an airplane goes
into soft or wet turf or the gear picks up debris (fig.
5), in addition to high-drag-load conditional inspections,
the wheel and tire assemblies should be replaced because water or
dirt may have contaminated the wheel bearings. Also, the wheel speed
transducers should be removed and inspected; brakes should be washed,
examined for obvious damage, and operationally checked; and the
entire gear should be cleaned of debris, especially under the axle
sleeves, and relubricated.

In one instance, an axle
fracture was attributed to moisture and mud under the axle sleeve
following an off-runway excursion. According to the maintenance
records, no inspection or cleaning was done, and the contamination
resulted in corrosion and crack initiation.

SUMMARY

Operators need
adequate data after a non-normal flight or landing or an off-runway
excursion to determine whether to conduct an unscheduled inspection
of the airplane. Operators are being provided information
to supplement flight crew reports of such conditions in the
Boeing AMM. This supplemental information includes vertical
acceleration thresholds for unusual landings, which will be
available in the AMMs for all Boeing- and Douglas-designed
airplanes later this year.

Also, more explicit
procedures for airline maintenance crews to use in initiating
conditional structural inspections will be added to the 737-100/-200/-300/-400/-500
AMM by third-quarter 2001. Boeing is standardizing these inspections
across all models to the extent possible, given differences
in structure that will require different inspections in some
instances.